Although this invention is applicable for measuring the wear on various irregularly shaped objects, it has been found to be particularly useful in the environment of PDC cutter elements. Therefore, without limiting the applicability of the invention to "PDC cutter elements", the invention will be described in such environment. This invention is also applicable for measuring the surface area of various substantially flat, irregularly shaped objects.
Polycrystalline diamond compact cutter elements are readily commercially available. Round top PDC cutter elements generally comprise a cylindrical stud of tungsten carbide having an angular planar face formed at one end of the stud. A generally cylindrical wafer of tungsten carbide is brazed onto the planar face with the opposite exposed face of the wafer comprising a layer of polycrystalline diamonds. It will be appreciated that the PDC cutter elements are available in a variety of shapes other than round top, such shapes as chisel top, flat top, etc. The cutter elements may be customized to almost any shape with an electrical discharge machine.
Sophisticated computer programs are used to accurately and efficiently position the PDC cutter elements on the drill bit so that each cutter element removes a predetermined volume of rock. As the cutter elements become worn, the drill bit becomes dull and the efficiency of the drill bit decreases until the dull drill bit is replaced with a new drill bit. The dull drill bit is sent back to design and manufacturing for evaluation. Wear data from the cutter elements is used to correct and adjust the mathematical models that are used to design bits and predict wear. Wear data is also used to determine if the bit is rerunable, or if the bit can be repaired, and if so, which cutters should be replaced.
It will be appreciated that it is very important and beneficial to be able to economically and accurately determine the amount of wear on each cutter element in the drill bit not only to obtain maximum and efficient use of the drill bit in the field prior to changing out the drill bit but also in optimizing the design of the drill bit and to make consistent decisions concerning repair and rerun of bits.
The present method of attempting to determine the amount of wear on each cutter element while the drill bit is in the field is by the use of a straight edge ruler having a millimeter scale thereon to attempt to measure a wear flat. This method is not very satisfactory because the millimeter scale readability is about 1 millimeter which introduces a great amount of error. In addition, there normally is not a flat portion on the wear surface of the cutter element as there are all types of surface geometry involved with the wear surface on each cutter element. Also, the vertical height of the remaining portion of the cutter may be attempted to be measured by the ruler and the same measurement error is introduced. Therefore, it is generally recognized that the ruler method is not accurate enough to provide the data needed to modify the mathematical wear patterns and the design programs for the drill bits. Additionally, scale measurement ignores the original positioning of the cutter in relation to the body of the bit. For example, a particular design may expose only 75% (by area) of a cutter above the body of the bit. Therefore, conversion tables or graphs must be used each time a measurement is taken to determine the true wear of the cutter as a percentage of its new condition.
A more accurate method of measuring wear of cutter elements is the ink grid method. Ink is placed on the face of the cutter element and then a small piece of paper is pressed against the inked surface to transfer the shape of the face of the cutter element to the paper. The paper is placed under a grid network and the number of squares and partial squares are counted and compared to the number of squares for a new cutter element to try and determine the amount of wear to the worn cutter element.
A more accurate and expensive method of measuring wear of the cutter elements is by use of a CMM machine which includes an element that is moved along the worn surface of the cutter element and data points (6 to 8) are taken. Sophisticated computer programs convert the information from the data points into an image of the cutter element and determines the amount of wear. It takes approximately 31/2 hours to do a 63/4 inch drill bit and the CMM machine is extremely capital intensive, time consuming, and expensive to use.
The present invention is portable, is relatively inexpensive, is extremely accurate and extremely fast. The wear of the cutter elements on a 63/4 inch drill bit can be determined in five minutes or less and the results will be more accurate than if the CMM machine was used. The data may be fed from this invention to a portable computer and taken back to the office.